1. Field of the Invention
The present invention relates to a piezoelectric ink recording device, and more particularly to an ink jet recording device that improves precision of where ink droplets impinge on a recording medium.
2. Description of the Related Art
There has been known an ink jet printer with an ink jet head including piezoelectric elements as actuators for ejecting ink droplets. FIG. 1 shows an example of such an ink jet head. The ink jet head shown in FIG. 1 is for ejecting hot melt ink, which is solid at room temperature and liquefies when heated. As shown in FIG. 1, the ink jet head includes a piezoelectric element 1, a diaphragm 5, and a nozzle plate 4 formed with a nozzle 4a. The diaphragm 5 is attached to one side of the piezoelectric element 1. The diaphragm 5 and the nozzle plate 4 define an ink chamber 3. The nozzle 4a is formed in the nozzle plate 4 at a position in confrontation with the diaphragm 5.
Although not shown in the drawings, the nozzle plate is formed with a plurality of nozzles 4a. The nozzles 4a are, for example, arranged in 32 columns and 12 rows, wherein the rows extend in the widthwise direction of the recording medium. The nozzle rows are divided into four groups of three rows each, each group being for one of four different colored ink types. That is, three rows each are designated for black, cyan, magenta, and yellow colored inks. An ink chamber 3 and a piezoelectric element 1 are also provided for each one of the plurality of nozzles.
Ink supplied from an ink tank (not shown) is temporarily held in a manifold 7, and then supplied to the ink chambers 3 through a corresponding ink channel 6. A heater 11 is provided adjacent to the manifold 7. The heater 11 heats the manifold 7 and maintains ink in a melted condition. A driver 10 is connected to the piezoelectric element 1. The driver 10 drives the piezoelectric element 1 in response to print commands from a controller 9.
FIG. 3 shows a configuration of the driver 10. The driver 10 is configured from a piezoelectric element driver 24 and a signal generator 25. A plurality of piezoelectric element drivers 24 are provided in a one-to-one correspondence with the piezoelectric elements 1a and 1b.
When an ink droplet 8 is to be ejected, the signal generator 25 outputs pulse voltage 2 shown in FIG. 4 having a pulse width W. The pulse voltage 2 is applied to the base of transistors Tr1 and Tr2 of the piezoelectric element driver 24. At this time, DC voltage 18 having a voltage level V is generated from the signal generator 25 and applied to the emitter of the transistor Tr1 and to resistors R1 and R2. As a result, a pulse voltage 20 having the pulse width W shown in FIG. 4 is applied to the piezoelectric element 1a. The piezoelectric element 1a deforms in association with the rising edge of the pulse voltage 20. The diaphragm 5 bends as indicated by a broken line 5a in FIG. 2. The volume in the ink chamber 3 increases in association with this, so that ink in the manifold 7 is drawn into the ink chamber 3 through the ink channel 6. Afterwards, the piezoelectric element 1 reverts to its initial shape in association with the falling edge of the pulse voltage 20. Accordingly, the volume of the ink chamber 3 decreases so that the ink droplet 8 is ejected from the nozzle 4a. On the other hand, when an ink droplet 8 is not to be ejected, the signal generator 25 is controlled so as not to generate the pulse voltage 2.
A laminated type piezoelectric element shown in FIG. 5 is capable of deforming the diaphragm 5 by a greater amount than other types of piezoelectric elements, so that the piezoelectric element can be driven with good energy efficiency.
However, in the above-described ink jet head, each of the piezoelectric elements has different properties for converting electrical to mechanical power. Also, different piezoelectric elements and corresponding diaphragms are coupled by different amounts and have different positional relationships. Because of these types of variation, the speed at which an ink droplet is ejected can vary depending on the nozzle. When more than one type of variation appears simultaneously in the nozzles, the problem of variation in ejection speed is compounded.
An ink jet head having the above-described variations can not print images with good quality. For example, when such a head is transported at a fixed speed across the width of a recording medium in order to print on the recording medium, the ink droplets can not be impinged at desired locations on the recording medium. The resulting printed image has poor quality. Also, the volume of ink in each ejected droplet can vary. Those nozzle that eject ink droplets with volume outside a certain range can be discarded at the factory in order to reduce variation in amount of ejected ink. However, this reduces the poor of ink jet heads.
The speed at which the ink droplets are ejected from a nozzle can be controlled by controlling a voltage to be applied to the piezoelectric element. Japanese Patent Laid-Open Publications Nos. HEI-4-310747 and HEI-9-39231 disclose methods for controlling charge and discharge currents for the piezoelectric elements. In Japanese Patent Laid-Open Publication No. HEI-4-310747, charge and discharge currents are controlled in the same manner for all of a plurality of nozzles. In Japanese Patent Laid-Open Publication No. HEI-9-39231, a charge pulse with a fixed voltage and a narrow pulse width is repeatedly applied to piezoelectric element circuits having a charge resistor and a discharge resistor. Based on how many narrow pulses produced the optimum printing results for various environments and ink types, a drive waveform for all of the piezoelectric element circuits is determined and stored in a ROM.
The methods disclosed in both of these Japanese Patent Laid-Open Publications uses digitally configured drive waveforms with a pulse width and voltage common for each of the plurality of nozzles. The drive waveform is not controlled differently for each of the nozzles.